The invention relates to a tool for the injection molding of synthetic plastics, with an inner mold and a two-part outer mold of which two substantially steel mold parts are adapted to be applied against each other in a sealingly tight manner along a plane of separation and moved away from each other again, said movements being brought about by control drives, the injection molding cavity being formed by a surface contour attachment disposed on the mold parts, cooling spaces being disposed behind the surface contour attachment on the side remote from the injection molding cavity.
For producing liquids packages, particularly for milk and juices (foodstuffs), it is already known for a top with a pourer orifice, and possibly already with a handle molded into place, to be injection molded onto a paper tube coated on both sides with synthetic plastics material because such liquids packages offer advantages to the manufacturer and also to the end user.
The injection molding of synthetic plastics tops into the paper tube takes place in the prior art paper producing machine by means of an inner mold which is constructed as a substantially cylindrical mandrel which, together with three other mandrels, is mounted on a mandrel wheel and is intermittently advanced during operation, one mandrel or one inner mold always being stopped during the injection molding process while, to form the injection molding cavity, two mold parts are moved together from outside so that they engage substantially at right-angles over the free end of the inner mold in such a way that they can be applied against each other in sealing-tight manner by corresponding control drives and pressed against each other and maintained in this position under pressure or possibly by other parts of the tool. The injection molding cavity formed in this way can be filled with the suitable synthetic plastics material under pressure so that the desired packaging part, e.g., the top or the top with an integrally cast handle, can be formed by injection molding.
From his practical experience, and from the literature concerning such injection molding machines, a man skilled in the art knows that the surface contour attachment is complicated and is comparatively expensive to produce, which is why especially the outer mold--and in this case each of the two mold parts--is expensive and should have a long effective life. But in the case of the two mold parts which form the outer mold, the stability on the one hand and the cooling on the other are important. Where injection molding is concerned, not inconsiderable pressures emanate from the synthetic plastics material in the injection molding cavity and they act outwardly on the boundary walls, namely the surface contour attachment. If the cooling spaces are too closely behind the surfaces which represent the surface contour attachment, then they can become deformed by the high pressure. For safety reasons, therefore, the cooling spaces are kept at a certain distance from the surfaces of the casting cavity. As the mold parts are made from steel, the heat conductivity from the cooling space disposed at a suitable distance from the surface contour attachment is, furthermore, limited. While it is possible to use milling tools on workpieces such as, for example, the mold parts, from outside (for instance, the complicated surface contour attachment), the cooling spaces are made by bores. It is a known fact, though, that bores cannot with any desired degree of accuracy be extended deep into the interior of a mold part. For this reason, too, it is advisable to keep the bores at a minimum safety distance from the surface contour attachment of the relevant mold part. The disposition of the cooling spaces in respect of the surface contour attachment on the one hand and the material of the steel mold part on the other lead to insufficient cooling, so that the output of the injection molding machines cannot be increased to the required extent.
A further drawback is the fact of general wear and tear in operation. Indeed, after a certain period of use, the entire mold part, together with its surface contour attachment, must be replaced by a new one in order to guarantee perfect moldings on the packages.